Cell senescence and apoptosis are thought to contribute to age-related diseases in mammals, and can be induced by critically shortened and thus uncapped telomeres. In humans, telomere shortening accompanies aging, and there is increasing evidence that shortening impairs tissue homeostasis. While maintenance of telomeres by telomerase is well understood, the roles of other telomere maintenance pathways are less well defined. The human Werner progeroid syndrome, caused by deficiency of the WRN RecQ-family helicase, is characterized by defects in telomere maintenance. Several lines of investigation, including our own work with mouse WRN and the S. cerevisiae homolog Sgs1, indicate roles for RecQ helicases in telomere maintenance by homologous recombination (HR)-based and G-quadruplex (G4) DNA-based mechanisms. Here we will investigate novel aspects of telomere maintenance involving RecQ helicases, G4-DNA, and HR. These will be investigated in three specific aims: 1) Determine the mechanisms by which deletion of the histone acetyltransferase encoded by SAS2 extends, in an HR- dependent fashion, the lifespan of telomerase mutants, 2) Investigate how the absence of Sgs1 enables cells lacking telomerase and homologous recombination to form survivors of telomere loss, and 3) Test the hypothesis that RecQ helicases can modulate telomere maintenance mechanisms via regulation of G4-DNA formation. These aims will be carried in S. cerevisiae, which provides an experimentally tractable model in which to dissect mechanisms, and findings from yeast will then be tested in mice and cultured murine and human cells. Mechanism will be explored through the use of established genetic, biochemical, pharmacological, molecular and cell biological approaches. Understanding how RecQ helicases, HR, G4-DNA, and histone acetyltransferases collaborate to regulate telomere maintenance will offer new insights into aging and cancer biology, and should provide new targets for the future development of therapeutics aimed at treating age- related diseases and malignancies.